The present invention relates to a process for causing gaseous trace elements, usually impurities, to pass from a flowing hot gas, such as industrial flue gases, into a dust by means of water.
Industrial gases, and specifically so-called flue gases, often contain gaseous trace element particles which, for one reason or another, usually for reasons of environmental protection, must be removed from the gases. Impurities can be removed from such gases by means of various processes and devices which have been described in the literature of the field. The processes are based on the physical or chemical absorption and adsorption of the trace element particles involved, by a liquid, a solid or a slurry. Typical examples of such processes include the absorption of hydrogen chloride, sulfur dioxide, sulfur trioxide and nitrogen oxides, present in flue gases, by alkaline compounds of calcium, magnesium, sodium or ammonium, and the adsorption of solvent vapors by, for example, activated carbon. Although the mechanisms of the phenomena have been studied in great detail in theory, the large gas amounts to be treated and often the weak driving force due to low concentrations have complicated the practical designing of apparatus and processes. This has resulted in several different known practical solutions to the problem.
In the known apparatuses in which gas is absorbed directly into a solid, the chemical efficiency of the absorption is in general rather low. When, for example, sulfur dioxide is absorbed into pulverous calcium oxide from the flue gases of power plants, the absorption rate is in general approximately 30%. This is due to the fact that diffusion on the reacting interface between the phases is, depending on the physical states, slow. Improvement of absorption for its part would quickly increase to unreasonable levels the size of the apparatus or the losses of pressure, or would otherwise require impracticable conditions.
In apparatuses in which the absorption takes place in the presence of a liquid phase, various deposits easily form, or the treatment, outside the actual absorption apparatus, of the liquids emerging from the apparatus requires complicated systems or presupposes a suitable existing integrated process. It also easily occurs that the liquids flow out of the absorption apparatus in an uncontrolled manner, causing various operational disturbances. If liquid is added in ample amounts, or if the liquid phase is continuous, the temperature of the hot gases falls to the wet temperature or below it, whereupon, in order to eliminate the risk of corrosion or clogging of the post-absorption gas-treatment apparatus, the gas has to be reheated. This implies either energy losses or, if it is carried out using hot gas which contains trace elements, a decrease in the overall rate of absorption.